This research investigates the importance of transitions among behaviors as the determinant of reinforcement, punishment, and contrast effects during operant discriminantion learning in pigeons. Contrast, which characterizes discrimination performance, suggests that the examination of transitions among behaviors may be more fruitful than the continued study of behaviors treated as isolated units. Such a view is compatible with historical accounts which laid the bases for much of current psychological theory, as well as with recent trends in other areas, such as sensory physiology. The Premack principle defines the reinforcement relation in terms of behavioral transitions but its use has been assumed to be restricted solely to the definition of reinforcement and (perhaps) punishment. Despite the popularity of this view, a number of problems have prevented its wider application. A larger amount of evidence suggests that a less restrictive version of Premack's definition is more useful; such an interpretation applies not only to reinforcement and punishment, but to aspects of discrimination learning as well.